1. Field of the Invention
This invention relates to navigation aids of the type which determine berthing data to assist the pilot of a ship during a berthing operation.
2. Description of the Prior Art
Such aids are in use to provide pilots, particularly the pilots of large ships such as super tankers and bulk ore carriers, with information as to the angle of the ship with respect to the jetty, the distance of the ship therefrom, and the speed of approach. Aids of this type are disclosed in U.S. Pat. No. 3,673,553, and U.K. Pat. No. 1,384,647. Installations are in use which provide a visual display of this data on the jetty within sight of the pilot, one such installation being described in the aforesaid U.K. Pat. No. 1,384,647, although alternatively the data may be transmitted by radio to the pilot on board ship. The data is derived from two distance detecting units normally empolying underwater sonar transducers and spaced apart so as to detect the distance from the jetty of respective bow and stern sections of the ship, normally referred to as the "fore and aft distance off" measured at bow and stern fenders on the jetty. Although such installations in general perform very satisfactorily problems have been encountered when, due to manoeuvring of the ship with resultant turbulence at the stern region or other causes, the signal from a detecting unit is materially affected or in some cases lost altogether.